Particularly in the calcination of materials in large lumps, such as for example limestone, dolomite or magnesite, with a feed particle size between 15-60 mm, rotary kilns are frequently used. In order to increase efficiency, shaft preheaters are often disposed upstream of these rotary kilns. Preheating and partial calcination of the lumpy material takes place in the shaft preheater using the exhaust gases from the rotary kiln. For an efficient heat exchange and in order to maintain an economical pressure loss there must be a good throughflow of the granular mass of material. Therefore the ratio between maximum and minimum fragment size of the feed particles is limited to 1:3. Thus for example a particle size spectrum of 15-45 mm or 20-60 mm can be treated in the shaft preheater. However, a greater particle size spectrum occurs in quarrying, so that the kiln system is operated correspondingly in stages.
In order to ensure a good throughflow of the granular mass of material it is necessary to adapt the layer height to the particle size spectrum of the material to be preheated. Thus a coarser particle size spectrum makes possible a greater layer height than a smaller particle size spectrum of the material to be preheated.
Therefore in order to process different particle size spectra it is known to provide means for limiting the layer height in the shafts. For example, it is known in the art for the covers of all shafts to be adjusted in height in order thereby to limit the layer height.
However, these known means for limiting the layer height are very expensive and, moreover, always only one particle size spectrum can be processed in the shaft preheater.
A shaft preheater which consists of kiln feed shafts disposed polygonally around a gas inlet shaft is known from DE-B-27 48 670. Each kiln feed shaft can have its own layer height control independently of the others.